THE SPINAL CORD 479 



This separation is increased by the growth into the cerebrum of 

 the centripetal fibre paths coming into the brain from the more 

 caudal parts. 



The grey matter of the cerebral cortex may thus be said to 

 represent a cephalic mantle or covering, which, by its stem of 

 nerve fibres, is supported above the cephalic end of a grey axis of 

 nerve centers which extends from the base of the cerebrum 

 through the midbrain and medulla oblongata to the spinal cord. 



This cerebro-spinal axis is developed in two symmetrical halves, 

 which are more or less completely separated from each other by 

 two series of deep sulci or fibrous septa, the one dorsal and the 

 other ventral. In the brain the dorsal is a deep sulcus, the ven- 

 tral a mere fibrous septum or raphe. In the spinal cord the con- 

 dition is reversed, the anterior or ventral median fissure forming 

 a deep sulcus, while the posterior is a shallow groove deepened 

 by a prominent median septum. 



The cerebro-spinal axis will be best appreciated by a study of 

 its several regions, in succession, from below upward. The fol- 

 lowing subdivisions are convenient for purposes of description : 



(1. Sacral region 

 2. Lumbar region 

 * rru /J ix 

 8. Thoracic (dorsal) region 

 4. Cervical region 

 a. Myelencephalon 5. Medulla oblongata 



( 6. PonsVarolii 

 BRAIN 

 (cepha- 

 lon) 



. 



b. Metencephalon j ?> Cerelbelum 



c. Mesencephalon 8. Region of the crurse cerebri (brain-stein) 



d. Diencephalon 9. Region of the optic thalami (basal nuclei) 

 ^ e. Telencephalon 10. Cerebral cortex (pallium) 



THE SPINAL CORD (Figs. 377 to 381). The spinal cord con- 

 sists of a considerable mass of central grey matter which is sur- 

 rounded by a layer of nerve fibres, the white matter. 



The grey matter consists of two lateral portions united by a 

 central commissure (grey commissure, posterior commissure). Each 

 lateral portion includes an anterior and a posterior horn with an 

 intervening deeper portion, the central mass or "intermediate 

 zone " of Golgi. 



The anterior is somewhat broader than the posterior horn. 

 Its cells supply neuraxes, which, after uniting into bundles, pass 

 ventral ward through the white matter to form the ventral (ante- 

 rior) nerve roots. 



